In recording and subsequently reproducing information in a system having relatively moving record medium and transducers, time base errors commonly occur in the reproduced information. For example, in quadruplex or other video recorder and reproducer devices having rotary transducers, information is recorded on and reproduced from separate tracks along the record medium and, when a plurality of transducers are employed, with switching occurring between the transducers as successive tracks are swept. In such devices, head to tape time base errors have long been recognized and appropriate means provided for compensating such errors.
Periodic or line by line organized information signals usually have periodically occurring synchronizing intervals, which ordinarily do not contain data information. Color television signals are common examples of such line by line organized information signals. A preponderance of television recorder and reproducer devices employ two or more transducers to transfer the television signal information between signal utilization equipment and a moving record medium. For example, in magnetic recorder and reproducer devices, a plurality of magnetic head transducers are commonly used to transfer television signal information from and to magnetic tape and disc media. In magnetic tape recorder and reproducer devices, a number of heads, typically four, translate across a moving magnetic tape in a transverse, helical or arcuate manner to produce a series of non-continuous recorder tracks. Upon reproduction, the heads move in similar fashion. As each head passes across the tape, it is switched into and out of the electronic signal processing channel. Quadruplex recorder and reproducer systems record the information in tracks transversely extending across a tape record medium, with each of the tracks containing several lines of information. Commonly each line is commenced by synchronizing information followed by data information. Any time base error in the form of misposition of a line is corrected by measuring the relative time base position of the synchronizing pulse of each line and applying an appropriate delay to each line to properly position the synchronizing pulses. In wideband signal applications, the intra line time base must be more accurately maintained. In color television signals, a pilot tone or color burst signal is contained in the synchronizing information and is used to provide precise time base correction of the wideband color television signal at the beginning of each of its lines.
Such time base correction does not, however, provide compensation for the effects of progressive time base errors that occur in each line from its beginning to its end. When recording and reproducing color television signals, the progressive time base error manifests itself as a change in hue across the television line. Because such errors are due to minute changes in the relative transducer to medium velocity as the head traverses the medium, they are commonly referred to as velocity errors. In one known method of velocity error compensation utilized in reproducing recorded color television signals, the velocity error for a given television line is determined by comparing the phase of the color burst of the line to be corrected with the phase of the next succeeding line. A discrete velocity error is then determined line by line for each individual line and a corresponding time base compensation effected. An example of such a velocity compensator is described in our application filed concurrently herewith entitled "TIMING ERROR COMPENSATOR" Ser. No. 554,886. In information recorder and reproducer devices that utilize the aforementioned method of velocity compensation, it is not possible to determine the time base error of the last line of information reproduced during each head pass prior to head switching because the time base related synchronizing information of the succeeding line will be reproduced by the next head to reproduce information from the record medium and will, consequently, not have a time base continuous with that of the synchronizing information reproduced by the prior head.
As used herein, "last line" or "last period" means the last determined period of a signal that is ordinarily reproduced by one of a plurality of reproduce transducers prior to the termination of the signal reproduction by such transducer.